Most telephone users are familiar with what is referred to in the art as the line or mounting cord which extends the telephone circuits from a connecting block, either floor or wall mounted, to a telephone set. The telephone set consists of the housing, and the handset which is connected to the housing by a retractile cord. It has been the custom to color match all of the aforementioned telephone components, that is, the line cord, the housing and handset and the retractile cord. Due to the large number of colors and the several different lengths of cords that are available, installers must maintain an uneconomically large inventory of line and spring cords on service vehicles in order to provide the many combinations of length and color. This large mobile inventory results in excessive inventory holding costs.
To reduce the excessive inventory holding costs, studies were made to ascertain if a single color line cord, which would significantly reduce the combinations, could be used. It was determined that this universal color must be neutral in any environment, compliment the telephone set, be aesthetically appealing and be widely accepted by telephone subscribers while also maintaining varied required physical characteristics. At first a satin silver colored polyvinyl chloride (PVC) jacket composition was selected as the universal color for line cords. However, the acceptability for this line cord was less than that which had been hoped for in a sample product test area.
Subsequently, it was decided to field test a line cord having tinned tinsel conductors individually insulated with a clear nylon material and jacketed with a clear PVC jacket. The test showed that a cord of this construction and color had aesthetic appeal as indicated by high acceptance rating in the product test area.
However, although the use of a non-flame-retardant, clear PVC composition solved the question of what universal color the consumer would accept, other problems arose due to the shortcomings of the clear PVC composition that had been employed. One problem that initially had to be overcome was to make a clear PVC composition which was flame retardant. Previously, flame retardation was accomplished by the addition of additives such as antimony trioxide which degraded the clarity of the PVC composition. Furthermore, for the line cord to be acceptable the composition must remain clear, that is, it must not degrade an exposure to ultraviolet radiation. These problems were the subject matter of U.S. Pat. No. 3,868,341. The composition described in that patent consists essentially of at least 80 percent by weight of polyvinyl chloride, 10 to 55 parts by weight per 100 parts by weight of the polymeric material of a phthalate plasticizer, 3 to 50 parts by weight per 100 parts by weight of the polymeric material of a flame retardant phosphate plasticizer, and 2 to 5 parts by weight of a metallic stabilizer together with other materials such as lubricants and ultraviolet absorbers and 1 to 4 parts by weight per 100 parts by weight of polymeric material of an epoxy resin, e.g., Epon.RTM. 828, together with 1 to 8 parts by weight of an epoxy plasticizer. This patent also describes, along with the flame retardant composition for line cores, methods of jacketing line cords therewith.
In order to act as a retractile cord, a certain flexibility is needed. The problem of flexibility of the spring cord has been dealt with in the past in U.S. Pat. No. 3,037,068. Other problems such as the tendency of skin oils to cause plasticizers to migrate from a PVC composition have also been dealt with in the art, for example, flexibility, mar resistance and oil migration resistance have been dealt with in U.S. Pat. No. 3,941,908.
A recent standard set for telephone cords is lack of vertical burn while still maintaining improved heat stability generally and in particular high humidity heat aging stability as well as low temperature flexibility, general flame retardation and clarity. It is also desirable to employ a composition which, if one desires, can be pigmented for the manufacture of telephone cordage which match in color to the handset and headset if desired. Vertical burn is tested by placing the telephone cord over a bunsen burner, in accordance with Underwriters Laboratories test UL-62, VW-1. To pass this test the cord should clear without burning.
The development of a suitable composition for jacketing telephone cordage is complicated by all the demands in requirements which telephone cordage must meet and because of the enumerable environments in which telephones are installed. Often, seemingly subtle differences in compositions can make the difference between meeting and not meeting certain requirements or the difference between commercial acceptance and not.